Communication systems are well known in the art. Generally, a communication system comprises a plurality of communication units, a communications resource controller and a pool of communication resources. In normal operation, the controller receives a service request from a communication unit, determines availability of communication channels and allocates, when available, a communication channel to the communication unit. For wide area communications, multiple sites are located throughout the area of coverage and linked to provide seamless communication between sites to a plurality of communication units. A multi-site system has common system parameters which are provided to the base stations at every site, such as operating frequencies and a system clock. The base stations may also be provided with a data link to communicate status information or data. Hence, in a multi-site system, all base stations are provided with three links, one for a frequency reference to generate the operating frequencies, another for a time reference to provide the system clock, and/or a third data.
A base station can generate operating frequencies using an integrated frequency generator, however, as frequency generators use frequency sensitive components, such as crystal oscillators, the generated operating frequencies may vary with ambient conditions. For example, base stations at different sites may be subjected to significantly different ambient temperatures that may result in substantial misalignment between the operating frequencies of the base stations. Consequently, when using affected base stations, communications may be degraded. In order to prevent misalignment, the frequency reference is provided to the base stations, limiting any variation of operating frequencies through out the system. Typically, the frequency reference is a 5 megahertz signal provided by a high stability reference frequency generator.
The time reference is provided to the base station to synchronize communication in the system, particularly when simulcasting digital information or when transferring data between communication sites. Typically, the time reference is allowed a maximum error of approximately one microsecond.
The data link is provided to the base stations to enable control equipment, such as a site controller, to determine the operating status of the base stations at a site, or to direct the base stations to execute self diagnostics tests and provide the results to the site controller.
At each site, separate co-axial cables may be used to distribute the frequency reference and the time reference, and a RS-232 cable may be used to provide the data, to the base stations. As a result, at each site, a substantial amount of cable is used requiring longer installation time for new sites, causing lower reliability due to the multiple connections, and providing a greater probability of radio frequency interference (RFI) attributed to the numerous lengths of cable carrying different signals.
Presently, a method of distribution used by the National Bureau of Standards on their WWV and WWVH channels, provides the frequency reference, the time reference and data in a single data stream signal. This is achieved by formatting the frequency reference, time reference and data into frames of one minute duration, and transmitting the frames at an effective data rate of one bit per second. A first bit provides a start-of-frame indication and the 59 bits that follow are pulse width modulated with data. This method can provide a frequency reference of one hertz with each bit, a time reference with the start-of-frame each minute, and data comprising 59 bits of information per minute. At the site of a radio communication system, however, this method imposes severe limitations. Firstly, data transfer only occurs at regular one minute intervals producing a slow data transfer rate. Secondly, the whole data frame incorporating the three pieces of information, has to be received and decoded to obtain any one of the three pieces of information. The design of such a decoder is relatively complex and may not be economical as each base station has to be equipped with the decoder irrespective of the base station requirements. For example, a first base station requiring only one piece of information is provided the same decoder as a second base station requiring all three pieces of information. In addition, this method is not backwards compatible with earlier base station designs that only require the frequency reference.
Accordingly, a need exists for a method that allows a frequency reference, a time reference and data to be individually extracted from a single data stream.